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Avalanche Forecast

Archived

Mar 6th, 2022–Mar 7th, 2022

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Coastal.

Strong to extreme northwest wind will continue to form wind slabs and grow cornices. Buried weak layers are at prime depth for human triggering and were reactive to skier traffic in the last few days.

Confidence

High -

Weather Forecast

Sunday night: Cloudy, up to 5 cm snow, extreme northwest wind, alpine low -4 °C, freezing level dropping to 600 m.

Monday: Mainly cloudy with sunny periods, up to 2 cm snow, moderate northwest wind, alpine high -5 °C, freezing level at 1000 m. 

Tuesday: Sunny, moderate to strong northeast wind, alpine high -10 °C, freezing level at 500 m. 

Wednesday: Mix of sun and cloud, moderate to strong northeast wind, alpine high -8 °C, freezing level at 300 m.

Avalanche Summary

There have been a few, skier triggered slab avalanches to size 2 on the two buried persistent weak layers described in the Snowpack Summary in the last few days. 

A skier triggered a large (size 2) avalanche on a north aspect in the alpine on Saturday. The avalanche released on a crust and was about 70 cm deep. 

On Friday, a natural large slab avalanche was observed on a northeast aspect that likely released on the buried surface hoar layer. Skiers triggered small (size 1) avalanches on a northeast aspect on the same layer. A natural cornice fall triggered a wind slab on the slope below and resulted in a size 2 avalanche.

A professional operation reported a slab avalanche being triggered by a wet loose avalanche from above on Thursday. And skiers triggered a size 1.5 slab avalanche from a distance away. It likely released on the buried surface hoar layer. 

Snowpack Summary

There's a lot going on in the upper snowpack. Careful assessment is recommended. 

There is a crust on the surface at lower elevations and on solar aspects at treeline.

Feathery, weak surface hoar crystals buried around February 26th are found down 25-50 cm, most prominently at treeline elevations. This layer has been reactive to skier traffic in the last few days. 

A thick crust buried in mid-February is now found down 60-100 cm. Skiers triggered avalanches on this layer in the last few days.  

The lower snowpack is effectively bridged by the layers above, and we do not expect avalanches to be triggered in the lower snowpack at this time. 

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid travelling on slopes below cornices.
  • Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
  • Be aware of the potential for large avalanches due to the presence of a buried crust.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.